The present invention concerns an improved process for preparing certain triaryl rhamnose carbamates.
U.S. Pat. No. 8,178,658 describes, inter alia, certain triaryl rhamnose carbamates and their use as insecticides. One of the methods used to prepare such triaryl compounds is by way of a the following 2 step process

wherein
R1 represents (C1-C6)haloalkyl or (C1-C6)haloalkoxy, and
R2 represents (C1-C6)alkyl, (C2-C6)alkenyl, or (C2-C6)alkynyl,
in which a triaryl acyl azide is converted to an isocyanate followed by reaction with a tetrahydropyran-2-ol and a strong base to give the triaryl rhamnose carbamate pesticide. However, the reaction of the triaryl carbamate with the tetrahydropyran-2-ol requires the use of a strong hydride base, provides an anomeric mixture of alpha and beta products, and gives rise to urea and aniline by-products. It would be desirable to have a process in which the triaryl acyl azide and the tetrahydropyran-2-ol could be coupled in good yield without the use of a hydride base. It would also be desirable to reduce the amount of by-product formation and to provide a predominance of the preferred anomeric isomer.